MnO2 Derived from LiMn2 O4

ABSTRACT

A new form of manganese dioxide having an x-ray diffraction pattern resembling none of the x-ray patterns of the known forms of manganese dioxide is manufactured by acid treatment of LiMn2O4.

This is a division of our prior U.S. application Ser. No. 947,120,filing date: Sept. 29, 1978, now U.S. Pat. No. 4,246,253.

This invention relates to a new form of manganese dioxide and a methodfor preparing the same. In particular this invention relates to a newform of manganese dioxide having an x-ray diffraction pattern notheretofore exhibited by any previously known forms of manganese dioxide.

This invention provides a novel form of manganese dioxide which is madeby acid treatment of the known material LiMn₂ O₄.

Manganese dioxide (MnO₂) is a well-known substance commonly used inelectrochemical cells, such as dry cell batteries, as an active cathodematerial. Manganese dioxide has been known to exist in variouscrystalline forms among which pyrolusite and nsutite are commonly foundin nature. Ramsdellite is also found in nature, but to a lesser extent.Other forms of manganese dioxide are known, either naturally occurringor man-made, but none of these alone or in combination has the x-raydiffraction pattern of the manganese dioxide of this invention.

LiMn₂ O₄, a spinel, is reported by Wickham and Croft (D. G. Wickham & W.J. Croft, J. Phys. Chem. Solids, 7, 351 (1958)) to form whenever lithiumcarbonate and any oxide of manganese are taken in a 2:1 molar ratio ofMn/Li, and heated at 800°-900° C. in air. The product LiMn₂ O₄ (a bluecolored material) contains equal amounts of Mn(III) and Mn(IV), andaccordingly has a manganese peroxidation value of 75% (% peroxidation isdefined as the degree to which the manganese oxidation state has beenraised from Mn(II) to Mn(IV). Thus, MnO has 0% peroxidation and MnO₂ has100% peroxidation). Wickham and Croft also reported that using excess Liin the reaction led to formation of a mixture of LiMn₂ O₄ and Li₂ MnO₃(a red material), while excess Mn led to a mixture containing Mn₂ O₃ inaddition to the LiMn₂ O₄. It should be noted that other preparativetechniques are possible for preparing LiMn₂ O₄, in addition to thosedescribed by Wickham and Croft. Other lithium or manganese compounds canbe used as starting materials provided they decompose to lithium ormanganese oxides under the reaction conditions used.

The novel manganese dioxide of this invention is made by acid treatmentof LiMn₂ O₄. The product of the acid treatment is a substantially pureMnO₂ whose x-ray pattern is nearly identical to that of the startingmaterial LiMn₂ O₄, a spinel. The x-ray pattern of the MnO₂ of thisinvention differs from that of LiMn₂ O₄ in that there is a slight shiftin peak positions, indicating a lattice contraction upon formation ofthe novel form of manganese dioxide, which we shall refer to hereinafteras "λMnO₂. " The "λ" designation, to the best of our knowledge, has notbeen used in the art to designate a form of MnO₂. Thus, while LiMn₂ O₄is a cubic spinel with a_(D) =8.24 A, λMnO₂ appears to have a closelyrelated structure with a_(D) =8.07 A±0.02 A. The term "a_(D) " as usedherein is the well-known term used in crystallography which refers tothe edge dimension of the cubic unit cell. A range of compositionsintermediate between LiMn₂ O₄ and λ-MnO₂ can be produced by controllingthe acid treating conditions, and such compositions can be representedby the empirical formula Li_(x) Mn₂ O₄ where O<λ<l, and have x-raypatterns with characteristics of both LiMn₂ O₄ and λ-MnO₂.

X-ray diffraction is a well-known and reliable test method for thedetermination of the structure of crystals. When a crystalline structureis bombarded with x-rays, some of the x-rays are scattered and changesin the phase relations between the rays scattered by different atoms inthe crystal result in a diffraction pattern characteristic of thespatial arrangement of the atoms in the crystal. The positions of thediffraction lines in a typical x-ray pattern are often referred to asd-values, indicated in Angstroms (A), and correspond to the planespacings in the bombarded crystal. These plane spacings and the relativeintensities of the lines are characteristic of the structure of a givencrystal. Identification of a substance by its x-ray diffraction patternmay be achieved by direct comparison with the patterns of knownsubstances, which is made easier through the use of published x-raypatterns classified in the card index of the American Society forTesting and Materials (ASTM).

For the determination of the x-ray patterns of the materials describedherein conventional powder diffraction techniques were used. Theradiation was Fe Kα, or Cu Kα, and a conventional scintillation counterdetector was used, with the resulting peaks displayed on a strip chartrecorder. d-values for the diffraction lines were calculated from theline positions and the wavelength of the impinging radiation, usingstandard tables.

The ideal starting material for preparing λ-MnO₂ is LiMn₂ O₄, which hasa Mn peroxidation value of 75%. However, in practice, it has been foundthat satisfactory results are obtained over a range of Mn peroxidationvalues, where the Mn/Li ratio in the starting mixture used to form LiMn₂O₄ varies somewhat from the ideal 2:1. As explained by Wickham andCroft, for Mn/Li of less than 2:1 (i.e. excess Li) some Li₂ MnO₃ forms.This is a distinctively red material, containing Mn(IV). It is notaffected by acid treatment, and it is of very low electrochemicalactivity. Even at a 2:1 Mn/Li ratio some of this material is often seen,probably due to incomplete reaction to form LiMn₂ O₄, because oflocalized variations of the Mn/Li ratio in the starting mixture. The useof a slight (up to 10%) excess Mn in the initial mixture used to formLiMn₂ O₄ tends to prevent formation of Li₂ MnO₃ and ultimately resultsin λ-MnO₂ of good activity. Thus, the optimum material for formingλ-MnO₂ is a LiMn₂ O₄ prepared in such a way as to be free of Li₂ MnO₃,and where some excess Mn₂ O₃ can be tolerated; where the peroxidation isin the range of 70-75%.

In accordance with the present invention there is provided a newcrystalline form of manganese dioxide with an x-ray diffraction patternhaving d-values of 4.64 A, 2.42 A, 2.31 A, 2.01 A, 1.84 A, 1.55 A and1.42 A, ±0.02 A in each instance.

In accorance with the present invention there is also provided a methodfor producing the manganese dioxide of this invention which encompassesacid-treating LiMn₂ O₄ under conditions specified in more detailhereinafter.

The treatment procedure typically involves suspending LiMn₂ O₄ in waterat room temperature by stirring, and then adding acid while continuingto stir and while monitoring the pH of the solution phase. Forsatisfactory conversion, on the order of greater than about 90% of theLiMn₂ O₄ to the λ-MnO₂ of this invention, acid treatment should continueuntil the pH of the solution phase stabilizes at below about pH 2.5,preferably below about 2. If a manganese oxide ore containing largeamounts of impurities is used as a starting material to make the LiMn₂O₄, then a more severe acid treatment, (i.e., stronger acid and/orhigher temperature) may be required to remove the acid-solubleimpurities while achieving the desired conversion to the λ-MnO₂.

The acids which are suitable for treating LiMn₂ O₄ in the practice ofthis invention include, but are not limited to acids such as H₂ SO₄,HCl, or HNO₃ ; other suitable acids selected by those skilled in the artmay be employed. The acids may be used in dilute concentrationsgenerally on the order of about 1 to about 10 normality.

The manganese dioxide of this invention is suitable for use in a numberof applications where manganese dioxide has been employed in the past.The most extensive use of manganese dioxide has been in electrochemicalcells, in particular in dry cell batteries, which typically comprise amanganese dioxide cathode, a zinc anode, and an aqueous electrolyte(such as aqueous NH₄ Cl and ZnCl₂ solutions). While the most widely usedelectrolytes in dry cells have been in aqueous form, the manganesedioxide of this invention is especially useful with non-aqueouselectrolytes, such as those comprising organic solutions of light metalsalts such as LiBF₄ in propylene carbonate-dimethoxyethane, or LiAsF₆ inmethylformate-propylene carbonate, and should also be useful with solidelectrolytes such as lithium-substituted beta-alumina.

In the drawing:

FIG. 1: is a graph showing discharge behavior of λ-MnO₂ in comparisonwith that of heat treated EMD in a non-aqueous electrolyte as explainedmore fully in Example IV hereinafter.

The following examples are set forth as being merely illustrative of theinvention and are not intended in any manner to be limitative thereof.Unless otherwise indicated, all parts and percentages are by weight.

EXAMPLE I

This example illustrates the preparation of λ-MnO₂.

20 gms of β-MnO₂ (reagent grade pyrolusite) were ground together with4.25 gms of Li₂ CO₃, heated in air to 835° C. for 10 minutes, cooled,and then reheated to 850° C. in air for one hour, and cooled to roomtemperature. The resulting reaction product was a blue powder found tobe substantially pure LiMn₂ O₄. About 15 gms of this reaction productwere placed in a beaker, about 400 mls water added, and while stirring,15% H₂ SO₄ was slowly added until the solution phase pH stabilized at 2.After allowing solid material to settle the supernatant liquid wasdecanted and the remaining solid was washed by decantation until thewash solutions were neutral. The solid was then collected on a sinteredglass filter funnel and dried in an oven at about 85° C. An x-raydiffraction pattern of the resulting dried product was obtained whichwas almost identical to that of LiMn₂ O₄, but shifted to lower d-values.Table 1 presents the x-ray diffraction pattern for the LiMn₂ O₄ preparedabove, and the above dried, acid-treated product. Also presented in theTable for comparison are data from the ASTM card 18-736, for LiMn₂ O₄ ascompiled by the "American Society for Testing and Materials." Chemicalanalysis, by conventional methods, of the acid-treated product indicatedthat the LiMn₂ O₄ had been converted to substantially pure MnO₂. Theresults of the chemical analysis are presented in Table 2.

                                      TABLE 1                                     __________________________________________________________________________              X-RAY DIFFRACTION d-VALUES AND RELATIVE                                       INTENSITIES Fe K∝ RADIATION                                  __________________________________________________________________________    LiMn.sub.2 O.sub.4 prepared                                                             4.73A(s)                                                                           2.48A(s)                                                                           2.38A(w)                                                                           2.06A(s)                                                                           1.89A(w)                                                                           1.58A(w)                                                                           1.46A(m)                              in Example I                                                                  Dried, acid treat-                                                                      4.64A(s)                                                                           2.42A(s)                                                                           2.31A(w)                                                                           2.01A(s)                                                                           1.84A(w)                                                                           1.55A(w)                                                                           1.42A(m)                              ed product of                                                                 Example I                                                                     ASTM card 18-736                                                                        4.72A(s)                                                                           2.47A(s)                                                                           2.37A(w)                                                                           2.05A(s)                                                                           1.88A(w)                                                                           1.58A(m)                                                                           1.45A(s)                              for LiMn.sub.2 O.sub.4                                                        __________________________________________________________________________     s = strong                                                                    m = medium                                                                    w = weak                                                                 

                  TABLE 2                                                         ______________________________________                                                 % Mn  % MnO.sub.2                                                                            % Peroxidation                                                                            % Li                                      ______________________________________                                        Theoretical for                                                                          60.77   72.12    75        3.83                                    LiMn.sub.2 O.sub.4                                                            LiMn.sub.2 O.sub.4 of                                                                    62.54   72.53    74        3.0                                     Example I                                                                     Dried, acid-                                                                             60.39   93.22    99        0.27                                    treated product                                                               ______________________________________                                    

EXAMPLE II

This example illustrates a preferred method of making LiMn₂ O₄ and alsoillustrates the use of different acids to convert LiMn₂ O₄ to the λ-MnO₂of this invention.

140.23 gms Li₂ CO₃ and 600 gms Mn₂ O₃ were ground together, heated inair at 850° C. for 1 hour; then cooled to room temperature, reground,then reheated at 850° C. for 1/2 hour.

Three 15 gm samples of the resulting LiMn₂ O₄ were placed in beakerswith 500 ml H₂ O, and treated with 3N HCl, 4.7N H₂ SO₄, and 4N HNO₃,respectively, to a pH of slightly below 2. The samples were then waterwashed until neutral, filtered and dried at ˜95° C. X-ray results (Table3) and analytical results (Table 4) confirm that in all cases the LiMn₂O₄ was converted to substantially pure λ-MnO₂.

                                      TABLE 3                                     __________________________________________________________________________              X-RAY DIFFRACTION d-VALUES AND INTENSITIES (Cu K∝                      RADIATION)                                                          __________________________________________________________________________    LiMn.sub.2 O.sub.4, prepared                                                            4.79A(s)                                                                           2.49A(m)                                                                           2.39A(w)  2.07A(m)                                                                           1.89A(w)                                                                           1.59A(w)                                                                           1.46A(w)                         in Example II                                                                 Dried, HC1                                                                              4.62A(s)                                                                           2.41A(m)                                                                           2.32A(w)                                                                           2.05A(w)                                                                           2.01A(m)                                                                           1.84A(w)                                                                           1.55A(w)                                                                           1.42A(w)                         treated product                                                               Dried, H.sub.2 SO.sub.4                                                                 4.64A(s)                                                                           2.42A(m)                                                                           2.31A(w)                                                                           2.05A(w)                                                                           2.01A(m)                                                                           1.84A(w)                                                                           1.55A(w)                                                                           1.42A(w)                         treated product                                                               Dried, HNO.sub.3                                                                        4.64A(s)                                                                           2.42A(m)                                                                           2.32A(w)                                                                           2.05A(w)                                                                           2.01A(m)                                                                           1.86A(w)                                                                           1.55A(w)                                                                           1.42A(w)                         treated product                                                               __________________________________________________________________________     s = strong                                                                    m = medium                                                                    w = weak                                                                      Note:                                                                         The additional weak 2.05 A peaks occurring in the acid treated products,      are probably due to tiny amounts of unconverted LiMn.sub.2 O.sub.4            remaining in the products.                                               

                  TABLE 4                                                         ______________________________________                                                        %    %       %                                                                Mn   MnO.sub.2                                                                             Peroxidation                                     ______________________________________                                        Theoretical for LiMn.sub.2 O.sub.4                                                              60.77  72.12   75                                           LiMn.sub.2 O.sub.4 Prepared in Example II                                                       60.36  70.91   75                                           Dried, H.sub.2 S0.sub.4 treated Product                                                         62.10  95.79   99                                           Dried, HCl treated Product                                                                      62.67  95.62   98                                           Dried, HNO.sub.3 treated Product                                                                62.08  95.55   97                                           ______________________________________                                    

EXAMPLE III

This example illustrates the use of an elevated temperature acidtreatment. 60 gms African MnO₂ ore containing about 74% MnO₂ was groundwith 10.3 gms Li₂ CO₃, then heated at 850° C. in air for 1 hour. Thesample was cooled, reground, then reheated at 850° C. for 1 hour more.The product showed the x-ray pattern of LiMn₂ O₄, and had 51.76% Mn,57.04% MnO₂, 70% peroxidation. The analytical results indicated thatimpurities were present, carried over from the ore.

Treating a 15 gm sample of the LiMn₂ O₄ product with H₂ SO₄ to a pH of 2resulted in a product which had 75.82% MnO₂, 92% peroxidation, and whichshowed the λ-MnO₂ x-ray pattern, indicating conversion to the new formof MnO₂. However, the low % MnO₂ seen in the analytical resultsindicated that the ore impurities remained high in the product.

A second 15 gm sample of the Li₂ CO₃ -treated ore was acid-treated,again to a pH of 2, but this time the treating solution was heated to90° C. and maintained at that temperature for 30 minutes prior towashing with water. The resultant product now had 84.3% in MnO₂ and 97%peroxidation and still showed the λ-MnO₂ x-ray pattern.

EXAMPLE IV

An 8.2 mg sample of λ-MnO₂ made according to the process described inExample II above, using H₂ SO₄, was placed on a porous nickel substrateand discharged versus a lithium anode in an electrolyte consisting of 1M LiBF₄ in 1:1 (volume ratio) propylene carbonatedimethoxyethane. Thecurrent drain was 500 microamperes. For comparison, a similar sized (7.8mg) sample of EMD (electrolytic manganese dioxide), heat treated for 8hours at 350°-360° C. to optimize its performance in the non-aqueouselectrolyte, was also discharged under the same conditions.

The results are graphed in FIG. 1; the horizontal axis is expressed inampere-hours per gram, allowing the normalization of the curves fordiffering sample weights. The vertical axis shows the cell voltagesduring discharge. The resulting discharge curves show that the λ-MnO₂has about the same total ampere-hour capacity as the EMD, but operatesat a much higher voltage for the first half of the discharge.

Although the present invention has been described and set forth in somedetail, it should be further understood that the same is susceptible tochanges, modifications and variations without departing from the scopeand spirit of the invention.

What is claimed is:
 1. An electrochemical cell comprising an anode, acathode, and an electrolyte, at least a portion of said cathodecomprising λ-MnO₂.
 2. The electrochemical cell of claim 1 wherein saidelectrolyte is a non-aqueous electrolyte.
 3. The electrochemical cell ofclaim 2 wherein said non-aqueous electrolyte is 1 molar LiBF₄ in 1:1(volume ratio) propylene carbonate-dimethoxyethane.
 4. Theelectrochemical cell of claim 3 wherein said anode is a Li anode.
 5. Theelectrochemical cell of claim 2 wherein said electrolyte is an aqueouselectrolyte.
 6. The electrochemical cell of claim 2 wherein saidelectrolyte is a solid electrolyte.